Understanding Surface Structure and Interactions of Ionic Liquids for Energy Applications

Henderson, Zoe (2019) Understanding Surface Structure and Interactions of Ionic Liquids for Energy Applications. Doctoral thesis, University of Central Lancashire.

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Presented within this thesis are four studies into the structure and interactions of ionic liquids (ILs), using X- ray photoelectron spectroscopy (XPS) both in situ and in vacuo, and using other complementary techniques. The findings provide insight into the surface chemistry and ordering of ILs, and are discussed in the context of energy applications.

The water/hydrophilic IL interface was investigated using near-ambient pressure XPS (NAPXPS) using a multilayer IL system (~109 Å) and an ultrathin layer system (~10 Å) on TiO2 substrates. Results indicate rearrangement of the outermost ions as water molecules adsorb on the IL surface, primarily manifested as intensity changes in the C 1s core level region. The higher binding energy features, associated with the charged parts of the IL (i.e. the anion and the imidazolium ring of the cation) increase in intensity with exposure to water, which infers a reorientation of the cation toward the interface. Because the water molecules were able to adsorb on the IL surface for a significant period under vacuum, this may have negative implications for IL catalysis systems, as water may inhibit the absorption of gaseous reactants.

The interactions between a superbasic IL and water/CO2 were investigated using NAPXPS. The reaction with CO2 forms carbamate, as evidenced by peaks in the N 1s core level region at the higher binding energy edge. The reaction is reversible by reducing the surrounding CO2 pressure. The results show that in each regime where the IL is exposed to CO2, the molar uptake ratio of CO2 molecules to IL pairs has an upper limit of 0.5. This indicates that the presence of water does not inhibit the IL’s ability to react with CO2 under near-ambient pressure conditions. Furthermore, it appears that the IL preferentially reacts with CO2 over water vapour. This has implications for gas capture and separation technology, where complex mixtures of gases, including CO2 and water, is released from industrial processes.

The structure and interactions at the IL/polar ZnO and IL/non-polar ZnO interfaces were probed using a combination of XPS and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Shifts in the core level XPS regions associated with the anion support the idea that the IL interacts more strongly with the polar ZnO than the non-polar ZnO. IL/substrate interactions are thought to be occurring at O-terminated step edges, which is a mechanism involved in other reactions on polar ZnO surfaces described in literature. As evidenced by the NEXAFS spectra, the imidazolium ring of the approximately submonolayer deposition on non-polar ZnO was found to orientate at an angle closer to the surface normal than the surface itself. This has important implications for solar cells, where molecular interactions at a surface can affect the charge transfer across the interface. The polar ZnO substrate appeared to somewhat catalyse IL decomposition at the surface, as determined by XPS measurements taken at different temperatures using an analogous IL on the same substrate. Signs of decomposition began to show at temperatures much lower than those in literature. This has consequences for solar cell applications, where thermal stability is important to maintain device longevity.

The electrochemical influence of ILs on the anodization of Ti was investigated using an IL-based electrolyte, and varying anodization voltages between 5 V and 20
V. Scanning electron microscopy revealed that nanoporous TiO2, and TiO2 nanotubes were synthesised. The surface chemistry, determined by XPS, revealed a trend with anodization voltage, which may be linked to electrochemical decomposition of the IL. The introduction of contaminant species into TiO2 nanotubes has consequences for their application in photocatalytic water splitting, where contamination could inhibit their hydrogen production capabilities.

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